1. Technical Field
The present invention relates to a projection-type display device and, in particular, to a projection-type display device suitable for a passive image creation device, exemplified by liquid crystals and/or by a digital micromirror device (DMD) that creates an image by moving a large number of small mirrors independently.
2. Background Art
The recent implementation of the promotion of the high-definition television has led to an increasing demand for large-screen display devices that can be used in ordinary homes. Although there has been progress in increasing the size of CRT devices, which have become standard as conventional display devices, to answer this demand, it is said that structural problems will limit the size thereof to 40 inches. As alternatives, it has become possible to produce plasma or liquid crystal display devices that are thin and have large screens, but these involve problems concerning reliability, lifetime, and particularly cost, so there is no basic solution for a large screen exceeding 60 inches.
Projection-type display devices, which project an original image produced by a small CRT or liquid crystal display device from a front plane or rear plane onto a screen in front of a viewer, have gradually appeared on the market as another method of implementing a large screen. In particular, improvements in compactness, resolution, and transmittance ratio in apparatuses that utilize liquid crystal display devices have astonished consumers and continue to act as a driving force in market diffusion. DMD apparatuses configured of tiny mirrors are also becoming practicable. These differ from CRTs in that they do not generate light in themselves, but are passive image creation devices that require separate illumination portions.
There are a number of technical problems that will have to be conquered to ensure an even wider market presence for a projection-type display device using such a passive image creation device. Representative problems concern: "brightness and uniformity on screen," "space efficiency," "cost," "weight," and "size."
The description below explains in steps the prior-art techniques used to solve the above problems. First of all, the basic configuration of a prior-art projection-type display device is shown in FIG. 36, to illustrate the technical problems concerning "brightness and uniformity on screen." This device is configured of an illumination portion 1 formed of a light source 1a and a parabolic mirror 1c for converting a principal light ray generated from the imaginary center thereof into a parallel beam; an image creation portion 2 formed of an image creation device 2a that selectively transmits luminous flux from the illumination portion 1 and a field lens 2b that controls the direction of the principal light ray; and an imaging portion 3 for projecting the original image created by the image creation portion 2 onto a screen 4. Reference number 3c of the imaging portion 3 denotes an aperture stop for controlling the brightness on screen and 3a, 3b, and 3d denote lenses.
With this optical system, the luminous flux reaching the screen 4 is reduced as the aperture stop 3c is stopped down and thus the imaging portion 3 reduces the cone angle .phi. from the image creation portion 2, so that the efficiency of utilizing luminous flux generated from the light source 1a is radically reduced. This is particularly dramatic when the light source 1a is large in comparison with the image creation portion 2.
Next, assume that the light source 1a is changed in a state in which the aperture stop 3c has been stopped down, in FIG. 36. This time, the efficiency of utilizing the luminous flux rapidly rises as the size of the light-generating portion of the light source 1a is gradually reduced. However, if it is assumed that the angle with respect to the optical axis, of the principal light ray generated from the imaginary center of the light source 1a, which is placed in the vicinity of the focus of the parabolic mirror 1c, is .omega., it becomes conspicuous that the brightness at the periphery of the image creation portion 2 drops, which is an inherent characteristic of a parabolic surface, in proportion to: ##EQU1## as to the angle .omega. increases (that is, with increasing distance from the optical axis). Therefore, when the size of the light-generating portion of the light source 1a is gradually reduced, the cone angle .theta. of luminous flux illuminating the image creation portion 2 can be reduced, and it becomes possible to increase the luminous flux until it is within the range of the cone angle .phi. of the imaging portion 3. On the other hand, this means that it is no longer possible to control the distribution of the cone angle .theta. of the luminous flux and the illuminance distribution in the image creation portion 2.
One means of solving this problem is to use an optical integrator (hereinafter abbreviated to "integrator") as can be seen in Japanese Patent Application Laid-Open No. 3-111806. This method was proposed with the aim of improving the illumination ratio of a conventional projector and eliminating irregularities in the degree of illumination and color of the image, as can be seen in U.S. Pat. No. 2,991,691, for example. The basic configuration thereof is shown in FIG. 37. Luminous flux from a light source 1a is guided by a reflecting mirror 1c into two lens arrays 1d and 1e that configure an integrator, then illuminates a film that configures an image creation portion 2. The luminous flux that passes through this film creates an original image and is projected by a lens of a imaging portion 3 onto a screen 4.
This integrator has two basic functions. In other words, these are an improvement in the efficiency of the luminous flux and an improvement in the uniformity of the brightness. First of all, an improvement in the efficiency of the luminous flux is achieved by a shape-modification function that modifies the circular luminous flux, which has been shaped by the initial reflecting mirror 1c, into the rectangular shape of the film. In an integrator of the most basic two-component configuration, the shape of each individual lens formed in the surface of an integrator on the light-source side, such as that of a lens 1da, is formed to approximate to the shape of the film to achieve this effect. This lens 1da can be considered to be a secondary light source, and a change of shape from a circle to a rectangle is caused by a lens corresponding to the lens 1da in an integrator 1e on the image creation portion 2 side, in other words, by a lens 1ea, to form an image on the image creation portion 2. The lens 1da also functions to create an image of the light source reflected by the reflecting mirror 1c onto the lens 1ea, and thus improve the coupling efficiency of the luminous flux between the lenses 1d and 1e.
The other action concerns uniformity of the brightness, which is achieved by sequentially superimposing secondary images of the light source on the image creation portion 2 by a large number of lens pairs, such as the lenses 1da and 1ea, in accordance with the same principle as that described above. In the above manner, the integrator is a superlative method for simultaneously achieving increased efficiency and uniformity, while having a simple configuration.
However, this integrator is not an universal solution and efficiency could be reduced by conditions imposed by the components configuring the projector. For example, Japanese Patent Application Laid-Open No. 3-111806 presented a number of proposals, based on considerations of the relationship between the size and efficiency of the light source. The basic concepts thereof are shown in FIGS. 38 and 39. There is fundamentally no great difference between FIG. 38 and the previously cited FIG. 37, but points that should be noted are the direction of the electrodes of an electric discharge lamp light source 1a, which is positioned perpendicular to the plane of the paper, and the use of an auxiliary imaging device 1b configured of a spherical mirror. This configuration is not particularly novel, being used in the prior art for overhead projectors and the like, but the combination thereof with an integrator deserves attention. A further step forward is illustrated in FIG. 39, where a portion configured of lenses 1c and 1d and reflecting mirrors 1e and 1f and a double condenser lens system configured of lenses 1c' and 1d' and reflecting mirrors 1e' and 1f' are used to collect the luminous flux from the light source. As a result, the cone angle of the luminous flux incident on an integrator 1g can be reduced and thus the efficiency of this luminous flux can be improved.
To ensure this improvement in efficiency, it is also important to keep the cone angle of the luminous flux at the lens 1d on the light-source side as constant as possible, in addition to the above described relationship between the size of the light source and the collection angle. Otherwise coupling losses will occur in the luminous flux between the lens 1d and the lens 1e. Japanese Patent Application Laid-Open No. 3-111806 provides absolutely no solution to this problem. It simply stops at making the size of each lens in the optical element 1e on the image creation portion side greater than the size of the light source formed thereon, when a conic surface is used for the lens 1c.
Japanese Patent Application Laid-Open No. 7-174974 discloses an example of the use of a reflecting mirror and aspheric lens between the light source and integrator, as means of solving the above described problem inherent to Japanese Patent Application Laid-Open No. 3-111806. A sectional view thereof is shown in FIG. 40. Luminous flux emitted from a light source 1a is guided onto an optical element 1e via a reflecting mirror 1c and aspheric lens 1d in such a manner that the cone angle distribution thereon is uniform. This was proposed to eliminate the coupling losses of luminous flux in Japanese Patent Application Laid-Open No. 3-111806. This example is configured with an awareness of the cone angle and direction control of the luminous flux. However, since it lacks any consideration of the relationships among the cone angle of luminous flux at the optical element 1e, the size of the light source 1a, and the size of the image creation portion 2, and it does not concern the collection angle from the light source as in Japanese Patent Application Laid-Open No. 3-111806, overall structural conditions ensure that the method disclosed in Japanese Patent Application Laid-Open No. 7-174974 cannot be used to construct the surface. This becomes especially obvious as the cone angle of the luminous flux at the optical element 1e becomes smaller, or the size of the light source 1a becomes relatively large in comparison with the size of the image creation portion 2.
To reduce the cost and increase the yield of image creation devices such as liquid crystal devices, it is most effective to design a smaller device and increase the number of elements that can be produced from a single wafer. However, this would be the cause of a huge decrease in the efficiency of luminous flux. For that reason, there is currently a great deal of work being done on increasing the illumination efficiency, starting with the above described integrators, but this is linked to an increase in the cone angle of the luminous flux in the image creation portion. Therefore, a lens with a small F number (a large cone angle) must be used in the imaging portion. It is not uncommon recently for a lens of F2 class (a cone angle of 29 degrees) to be used, which increases the cost of the imaging portion.
In contrast to this use of a lens with a small F number, some image creation devices necessitate the use of an imaging portion with a large F number. Intense interest has recently been focussed on a display device using a polymer dispersed liquid crystal, as a means for improving the brightness of a liquid crystal display device. A major feature of this display device is the use of the scattering characteristic of a liquid crystal sealed into a small region to make the image visible. This differs from the usual method in which the polarization characteristic is utilized so that the image is made visible with the aid of a polarizer, and has the advantage that the lack of this polarizer can be expected to improve the brightness by an equivalent amount. This concept is illustrated in FIGS. 41 and 42. Note that, although these figures show a transmissive system, the principles are exactly the same for a reflective system. FIG. 41 shows a transmission mode and FIG. 42 shows a scattering mode. In these figures, when luminous flux f is incident on a polymer dispersed liquid crystal, in the transmission mode it is transmitted therethrough unchanged, but in the scattering mode it forms scattered light with a substantially circular diffusion pattern. By switching between these two modes an image is created utilizing changes in light quantity within a region of a specific solid angle.
From the above principle, how the scattered light that reaches the screen in scattering mode is reduced is a decisive factor in improving the contrast. For that reason, an optical system called a Schlieren optical system is employed in the imaging portion 3, but this has a disadvantage in that the F number is large. In other words, this means that a high contrast is obtained by reducing the cone angle .phi. of the collected luminous flux, thereby reducing the scattered light that reaches the screen. Therefore, although this device has an advantage in being brighter because there is no need for a polarizer, this is cancelled by the stopping down achieved by reducing the cone angle .phi. of the imaging portion 3, so there is a danger that in fact the image will end up even darker.
If, as an experiment, the F number of the imaging lens that is usually used is made to be approximately 4 and the most suitable F number for ensuring contrast with a polymer dispersion device is 11, the brightness is reduced to 1/8. The effect of removing the polarizer is at most approximately 1/3, so it is clear that the brightness could be reduced to one half or less, as feared.
An optical system using such a polymer dispersed liquid crystal, as disclosed on page 113 of Japan Display '92, is shown schematically in FIG. 43. In this example, a metal halide lamp with a gap of 5 mm is used as a light source 1a and light that has been focussed once by an elliptical mirror 1c is converted into a parallel beam by a collimeter lens 1d. A 3.4-inch polymer dispersed liquid crystal is used as an image creation device 2a, and, after the light has passed through a field lens 2b, it is projected onto a screen 4 by a lens system 3 with an F number of 9.5 (a cone angle of 6 degrees). If a light source 1a of 150 W is used, the luminous flux on the screen is 400 lm.
The illumination portion of FIG. 43 is configured of the single elliptical mirror 1c alone, as described above, but, if the amount of light output per watt of the light source is 2.6 lm/W, a comparatively high efficiency will be exhibited. This is because a liquid crystal panel 2a that configures the image creation portion 2 is comparatively large at 3.4 inches and the light source 1a is relatively small, so that the cone angle .theta. of luminous flux on the panel can be reduced. However, if it is decided to reduce the size of the panel to reduce costs, it is no longer possible to maintain this high efficiency by the elliptical mirror alone. This is because there are technical limits to reducing the size of the light source 1a together with the panel size, which result in an increase in the cone angle .theta. of luminous flux incident on the panel 2a, bringing about the previously described problem concerning the F number.
The description now turns to a consideration of the prior-art techniques for solving the second of the technical problems relating to the popularization of projection-type display devices: "space efficiency." The best kind of projection-type display device is of a front projection type that projects an image onto the screen from the same side as that of the viewers. Unfortunately, this type of display device has a large disadvantage in that it takes up part of the space that ought to be occupied by the viewers. Several methods have been used to counter this problem, such as mounting the projector on the ceiling, but the overall cost of extras such as reinforcement of the ceiling is one of the reasons why the front projection type is not popular.
The same type of projector can also be used as a rear projection type, as shown in FIG. 44. In this case, luminous flux from a projector 7a is bent by plane mirrors 7b and 7c, then is projected onto a screen 4. With this rear projection type, it is necessary to shorten the focal distance of the imaging portion in order to reduce the depth of the display device itself. This is equivalent to employing a wide-angle lens in the imaging portion. In the same way as with a pickup element, it might be considered to shorten the focal length of the camera lens that is used, together with reducing the size of the light-receiving portions. However, with a passive image creation device, there is the usual problem with the size of the light source so that there is a limit on the relationship with the efficiency of the luminous flux. Thus it is currently difficult to make the device even more compact.
An optical projector has been proposed in Japanese Patent Publication No. 6-1295. The basic configuration thereof is shown in FIG. 45. This optical projector is provided with a light source 1a having a small light spot, such as a xenon lamp, and reflecting mirrors 1c and 1d providing control over the luminous flux to reproduce a secondary point light source 1a'. In this case, 1b denotes an auxiliary imaging device that operates in the same way as the auxiliary imaging device 1b of FIG. 38 and 1e denotes a flat reflecting mirror that simply bends the luminous flux back. By placing an image creation portion 2 between the thus created secondary point light source 1a' and a screen 4, which is not shown in the figure, an image can be projected onto the screen by the shadowgraph principle. The main purpose of the luminous-flux controlling reflecting mirrors 1c and 1d is to create the secondary point light source 1a' for creating an image of uniform brightness, from consideration of the luminance distribution and luminance intensity distribution of the lamp and differences in distance between each part of the image creation portion 2 and the secondary image on the screen. In other words, their main purpose is to control the illuminance distribution and direction of luminous flux generated from the light source.
Among the main features of this optical projector are four features that make this projector extremely useful: 1) it is possible to project an expanded image from a short distance, 2) oblique projection is possible, 3) uniform brightness can be guaranteed over the screen, and 4) the deep focal depth enables focusing at any place (focus-free). If it is possible to use such a projector, the projection can be from a position at which the viewers are not inconvenienced and the space efficiency problem can be solved at a stroke.
The measures used to solve each of the above technical problems tend to increase the cost of the entire apparatus, which has an effect on the third technical problem relating to the projection-type display device. For example, a microlens array could be employed in the liquid crystal display device that is one of the main components of a passive display device, to improve the transmittance ratio. Similarly, further increases in cost are caused by employing a special illumination system comprising components such as a polarizing prism and an optical integrator to improve the efficiency, and increasing the number and size of lenses in the lens assembly and also employing a dichroic prism to reduce the F number of the imaging lens and reduce the focal distance. Therefore, even if the image creation device is made more compact to reduce costs, that will be offset by the measures used to improve the illumination efficiency.
An example of the use of a reflective optical system in the imaging portion 3 is shown in FIG. 46, as a final example of a prior-art projection-type display device. In this example, a CRT is used as the image creation portion 2. A CRT is a representative example of an active image creation portion that generates its own luminous flux. The imaging portion 3 consists of a reflecting mirror 3a and a correction plate 3b, to configure a system called a Schmidt system. This imaging portion 3 is employed to increase the efficiency of this luminous flux and make the projected image brighter, because the luminous flux from the image creation portion 2 is divergent light, but the resolution is not very good. The advantages and disadvantages of this configuration have already been made clear in the general literature on the Schmidt camera, so they are omitted herefrom. Even now, it is for the purpose of improving the efficiency of luminous flux having a wide dispersion angle that a lens system with an F number in the 1.0 class is used in a projector utilizing a CRT.
The effective obverse of the large aperture of the reflective system utilized in the imaging portion 3 of FIG. 46 is a difficulty in obtaining a large angle of view. That is why it is mainly used in applications such as large-aperture telescopes. In recent projection-type display devices, as touched upon in the example of the rear projection type, there is an increasing tendency for the imaging portion to have a large angle of view, and thus it is difficult to adopt such a configuration. However, this suggests that it is possible to simplify the optical system by using a reflective optical system.
As described above, the various technical problems of "brightness and uniformity on screen," "space efficiency, " "cost," "weight," and "size" must be solved in a comprehensive manner, to enable the projection-type display device to achieve wide market acceptance as a large-scale display device. As should be clear from the above discussion of the prior-art techniques, not only is it inevitably necessary to improve the individual technologies, we should go further and consider that we are approaching a time at which we should basically rethink the configuration and purpose themselves, in order to solve such intricate problems.
In this context, the optical projector of the previously mentioned Japanese Patent Publication No. 6-1295 has four extremely good features. However, implementation of this projector involves a number of problems. These can be summarized as two main problems. One is the problem of resolution; the other is a problem concerning mass production. First of all, concerning the problem of resolution, theory states that the effects of half-shadows and diffraction are obvious in the creation of a secondary image by the shadowgraph principle from luminous flux from a light source that is close to being a point source. Thus it is not possible to respond to demands for a resolution that exceeds a certain level. This problem is caused by the lack of an imaging portion in the above optical projector. Two particular problems concerning mass production that can be raised are: "scratches and unevenness in the reflective surfaces used affect the final image" and "shaping accuracy of the reflective surfaces is strict." Conquering these will require an extremely high level of machining accuracy, which is the biggest problem from the mass-production viewpoint. Both of these are also common reasons for making the light-generating portion of the light source extremely small. In contrast thereto, there is also a conflicting problem in that it is difficult to obtain a "light source having a small, bright light spot," to improve the brightness and resolution.
The present invention was developed with the objectives of solving each of these technical problems and satisfying all of the conditions that ought to be provided by a projector, while keeping the advantages of the optical projector of Japanese Patent Publication No. 6-1295. In other words, the objective of this invention is to simultaneously achieve both the technical and manufacturing conditions of:
(1) Implementing the four major features of a magnified projection over a short distance, oblique projection, a uniform illumination, and a deep focal depth, with a bright, high-resolution image. PA1 (2) Providing a compact, light-weight, and inexpensive projection-type display device.
To summarize the technical fields that the present inventor has returned to and fundamentally reconsidered in this case, I have clarified the relationships among the characteristics of the light source, the cone angle of the luminous flux, and control over the luminous intensity distribution, illuminance distribution, and directionality thereof as well as the dimensions of the image creation portion, relating to the implementation of the features listed in (1) above, and have determined a basic way of considering how to construct the illumination portion in practice. These make it possible to implement optimization of the disposition and surface shape of the various optical elements configuring the illumination portion in accordance with the dimensions, shape, luminance distribution, and luminance intensity distribution of the light-generating portion of the light source, and the dimensions and characteristics of the image creation portion.
A reflective optical means is used in the imaging portion, and, as a further step, a configuration has been enabled in which the imaging portion has only a few reflecting mirrors, to implement the features listed in (2) above. In a prior-art projector, it is difficult even just to concentrate on brightness by configuring the imaging portion having such characteristics with only a few mirrors. As described above, the object of the present invention is to implement an ideal projector that satisfies the conditions listed in (1) and (2) above, by implementing a highly efficient, highly sophisticated illumination portion by suitable utilization based on a comprehensive overview of the various luminous flux control techniques, and applying this illumination portion to satisfying the conditions imposed on the imaging portion.